1. Field of the Invention
This invention relates to an apparatus for effectively erasing a residual image on a stimulable phosphor sheet which is used repeatedly. This invention particularly relates to an apparatus for effectively erasing a residual image on the stimulable phosphor sheet repeatedly used in a radiation image recording and reproducing system wherein the stimulable phosphor sheet is exposed to a radiation to have a radiation image stored thereon and then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, the emitted light is detected and converted into an electric image signal, and the electric image signal is processed and used for reproducing a visible image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the radiation energy stored. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). The stimulable phosphor sheet used in the radiation image recording and reproducing system may be in any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For reasons of economy, it is desirable that the stimulable phosphor sheet be used repeatedly.
Theoretically, when the stimulable phosphor sheet is exposed to stimulating rays of a sufficient intensity to cause the sheet to emit light in proportion to the stored radiation energy at the image read-out step, the stored radiation energy should be released completely and disappear. Actually, however, since the intensity of the stimulating rays used at the image read-out step is not sufficient to completely erase the stored radiation energy, a part of the stored radiation energy remains unerased in the stimulable phosphor sheet, so that when the sheet is used repeatedly, the unerased radiation energy causes noise to arise in the visible image reproduced from the reused sheet.
As described, for example, in U.S. Pat. No. 4,400,619, the aforesaid residual image can be erased by stimulating the stimulable phosphor sheet by light having a wavelength within the stimulation wavelength range for the stimulable phosphor constituting the stimulable phosphor sheet before the next image recording on the stimulable phosphor sheet, thereby sufficiently releasing the stored radiation energy. In order to completely erase the residual image, the light exposure amount (i.e. illuminance.times.time) should be adjusted to as large a value as possible. However, from the viewpoint of decrease in energy requirement and erasing time, and service life of the erasing light source, it is desirable that the light exposure amount be limited to the minimum necessary value. In order to satisfy the two incompatible requirements, the applicant in U.S. patent application Ser. No. 440,046 (now U.S. Pat. No. 4,584,482) proposed detecting the residual radiation energy level stored on a stimulable phosphor sheet and adjusting the erasing light exposure amount in accordance with the detected residual radiation energy level.
By controlling as described above, it is possible to adjust the light exposure amount to a necessary, sufficient value for each stimulable phosphor sheet, and to satisfy the two incompatible requirements described above for each stimulable phosphor sheet.
However, there are a portion of a high level of stored radiation energy (and consequently a high level of residual radiation energy) and a portion of a low level of stored radiation energy (and consequently a low level of residual radiation energy) on the one stimulable phosphor sheet. Therefore, in the aforesaid method, a portion of the sheet is exposed to an unnecessarily large light exposure amount.